Lithium ion batteries (hereinafter referred as “LIB”) are widely used in various kinds of electric appliances, and also used as power energy of electric vehicles, due to the LIB having advantages of higher operating voltage, higher energy density, stable discharge potential, low self discharge, long cycle life, no memory effect and no pollution.
When LIB is used as power energy of electric vehicles, the requirement of power energy for LIB is much higher than portable electric appliances. In addition to improve cathode material, electrolyte and separator, the improvement of anode material is also important. Selection of an anode active material is a key factor that affects the performance of LIB. The anode active material used in existing commercial LIBs is mainly graphite, which has a low insertion potential and an excellent lithium insertion/deinsertion performance. Thus, the graphite is a good anode active material for LIB. The capacity of insertion/deinsertion of lithium ions in graphite can be carried out according to chemometry of LiC6, the theoretical capacity of graphite is reached to 372 mAh/g, but the practical capacity of graphite is generally 330 mAh/g, which is quite close to its theoretical capacity. Thus, it is difficult to further improve the capacity of graphite.
The low capacity of carbon-based anode active material (e.g., graphite) restricts the energy density of LIB. Therefore, some non-carbon anode active materials have attracted attention by the industry because of higher energy density. Among them, silicon is a potential anode active material for the upgrading of graphite, wherein the nominal capacity of silicon can be reached to 4200 mAh/g, which is much higher than graphite. Also, the voltage plateau of silicon is higher than graphite, therefore it is not easy to arise lithium plating during charging and with better safety performance as well. However, the cycle performance of silicon is not good enough, and the volume change of silicon during lithium insertion/deinsertion is huge and can be reached to 300%. The volume change effect may separate the anode active material from the current collector. Also, silicon by itself is prone to chalking, resulting in a decline in battery performance. In addition, silicon is a semiconductor material, its conductivity is low.
In order to overcome the disadvantages of silicon-based anode active material, a lot of works have been done by researches on silicon. For example, the silicon material is processed to nanoparticles silicon, porous silicon, or coated silicon, wherein coating examples typically include coating carbon on silicon or coating inert material on silicon. When the silicon is coated to form a composite material, silicon is the main part of the composite material, the outer layer coated on the silicon can be used to buffer volume expansion and increase electron transport ability. By coating a layer of carbon on outer surfaces of silicon nanoparticles, a silicon carbon composite material having a core shell structure is obtained. If the size of silicon particles is at nanoscale, the volume change effect is small, and the existence of a layer of carbon on the silicon will decrease direct contact between the silicon particles and the electrolyte and improve electron transport capability among the silicon particles, to enhance cycling stability of the whole anode electrode.
Chinese patent application No. 200510119964.9 discloses a silicon composite consisting of silicon particles whose surface is at least partially coated with a layer of silicon carbide. The size of the silicon particles is in the range of 50 nm to 50 μm, and an outer surface of the silicon particles is sintered at least partially to form a layer of silicon carbide. By means of coating the silicon particles with a layer of the silicon carbide, the initial efficiency and the cycling stability are improved, the volume change during charging and discharging is reduced, making the silicon composite more suitable for LIB anode active material.
Currently, silicon coating is mainly realized by coating a layer of inert material or carbon material on outer surfaces of silicon particles. The outer coating layer can control the volume change effect of silicon during charging and discharging. Thus, the problems of great volume change during charging and discharging and easily being chalking can be solved, to a certain extent, by coating the silicon particles. However, because silicon as an anode active material is coated and covered by carbon material or inert material, lithium ions have to pass through the outer coating layer firstly and then embed into the anode active material (i.e., the silicon) in the process of lithium insertion. Therefore, the outer coating layer outside the silicon will affect the rate capability of the anode material.